EE535: Renewable Energy: Systems, Technology &

EconomicsBioenergy

Introduction

• Energy derived from ‘sustainable’ materials. Cellular matter from living or recently dead organisms– Wood (short rotation forestry)– Straw (miscanthus grass,…– Waste (industrial residues, recycled wood, manure,

organic domestic waste, sewage sludge,..)

• Material can be converted directly to produce heat an power

• Material can be converted into Biofuels (e.g. biodiesel, charcoal)

Biomass

• All the earth’s living matter is called its biomass, and exists in the thin surface layer called the biosphere

• Biomass is only a small portion of the earths mass, but in human terms is an important energy source

• The biomass is constantly being replenished by the flow of energy from the sun, through the process of photosynthesis

Photosynthesis• Photosynthesis is the making (synthesis) of organic structures and

chemical stores by the action of solar radiation• Solar radiation on green plants and other photosynthetic organisms

must relate to 2 dominant functions:– Temperature control for chemical reactions to proceed– Photo excitation of electrons for the production of oxygen and carbon

structural material• Maintaining correct temperature is important, so solar radiation

might be reflected or transmitted, rather than absorbed to increase photosynthesis

• The main organic material produced is carbohydrate (e.g. glucose C2H12O6)

• If we burn glucose in Oxygen, the heat released is about 16MJ/kg (4.8eV per carbon atom, 470KJ per mole of carbon)

Photosynthesis

• Basic process for fixation of atmospheric CO2 to carbohydrate :– Reactions in light: photons produce O2 from

H2O, and electrons are excited to produce strong reducing chemicals

– Reactions without requiring light: reducing chemicals reduce CO2 to carbohydrates, proteins and fats

CO2 + 2H2O Light O2 +[CH2O] + H2O

Photosynthesis

• [CH2O] represents a basic unit of carbohydrate, so the reaction for glucose is:

6CO2 + 12H2Olight

6O2 + C6H12O6 + 6H2O

Leaf

LightReaction

DarkReaction

Roots

SolarRadiation

O2

CO2

O2CO2

H2O

H2O

Chemical Exchange

Bioenergy Conversion Technologies

1. Combustion

2. Anaerobic Digestion

3. Gasification

4. Pyrolysis

Combustion

• Biomass (e.g. wood chips) can be burned to provide process and/or space heating.

• The combustion of biomass can also be used to raise steam to drive engines / turbines which are coupled to generators producing electricity.

Anaerobic Digestion

• The process of Anaerobic Digestion (AD) involves the breakdown of organic waste by bacteria in an oxygen-free environment

• Products of this process are Biogas and agricultural fertilizer (rich in nitrogen, phosphorous, potassium, ..)

• Biomass (e.g. animal manure) can be transformed to biogas by anaerobic digestion and the biogas can be used to fuel a gas engine or gas turbine, or burned in a boiler to provide heat or to raise steam.

• Biogas is a combustible gas composed primarily of Methane (CH4) and CO2

The sustainable cycle of biogas from anaerobic digestion

http://www.big-east.eu/downloads/IR-reports/ANNEX%202-39_WP4_D4.1_Master-Handbook.pdf

Biochemical Process of Anaerobic Digestion

inefficiency

H2

Typical Biogas Composition

Gasification• Chemical processes by which a

gaseous fuel is produced from solid fuel

• Usually the raw material, such as house waste, or compost are heated to a high temperature (>700C) with a controlled atmosphere of oxygen and/or steam

• Process begins with the release of volatiles from the heated solid, leaving the char

• These components undergo reactions with steam and oxygen to produce ‘Syngas’ or a ‘producer gas’ – mainly carbon monoxide and hydrogen, but includes other trace gases

C + H2O → CO + H2 C + O2 → CO2 CO2 + C → 2CO

http://energy-squared.com/images/gasification_schematic.jpg

Pyrolysis

• Collection of volatile components generated when a raw material is heated and condensation to produce a fluid – Bio-oil

• Method: heating (not burning) the bio-material with a carefully controlled air supply, minimising gasification

Technology Status for Power Generation from Biomass

SEI Briefing Notes on Biomas

Technology Status for Heat Generation from Biomass

SEI Briefing Notes on Biomas

How Much Energy Can You Actually Harvest from Bioenergy?

This figure illustrates the quantitative questions that must be asked of any proposed biofuel:

What are the additional energy inputs required for farming and processing? What is the delivered energy? What is the net energy output?

Often the additional inputs and losses wipe out most of the energy delivered by the plants

http://www.inference.phy.cam.ac.uk/withouthotair/c6/page_45.shtml

Biomass Calorific Values

Moisture kWh / kg kcal / kg Weight kg / m3 kg = 1liter fuel

Bark fir 50% 2,14 1,84 280 4,65Briquettes 20% 4,9 4,214 660 2,03Forest wood chip dry 40% 2,89 2,511 240 3,44Forest wood chip fresh 55% 2 1,72 310 4,98Miscanthus 10% 4,4 3,78 140 2,26Rapeseed 9% 6,83 5,87 700 1,46Sawdust 6% 4,2 3,629 160 2,36Stover rapeseed 15% 4,17 3,58 115 2,43Sunflower 9% 5,56 4,78 600 1,79Wheat  15% 4,17 3,58 700 2,4Wheat Straw 15% 4 3,44 100 2,49Wood chip 20% 4,22 3,629 175 2,36Wood granulate 8% 4,44 3,81 600 2,24Woodlogs ash  45% 2,61 2,245 650 3,81Woodlogs ash dry 20% 4,08 3,509 400 2,44Coal 10% 7 6,02 750 1,36Fuel gasoil 11,8 11,2 840 0,84Natural gas 10,83 9,314 0 0

See also : http://www.bkc.co.nz/Portals/0/docs/tools/calorific_value_calculator.html

Most fuels are not oven dry when burnt and the water in the fuek must be evaporated, detracting from the extractable energy* (or net calorific value). Moisture or water content is the single biggest factor in the variability in combustion behaviour of biomass

http://www.biofuelsb2b.com/useful_info.php?page=Typic

Petroleum Substitution

• Biodiesel from Rapeseed– Circa 1200 litres of oil produced per hectare– Energy density = 9.8kWh per litre– So, power per unit area is 0.13W/m2

• Sugar beet to ethanol– Circa 53t of sugar beet per hectare per year– 1 t of sugar beet yields 108 litres of bioethanol– Energy density = 6kWh per litre– So, power per unit area is 0.4W/m2

Petroleum Substitution

• Bioethanol from sugar cane– 80 t per hectare can be produced in the right climate,

yielding about 17600 litres of bioethanol– Energy density of 6kWh per litre– So, power per unit area is 1.2W/m2

• Biodiesel from Algae– Water can be heavily enriched with CO2

– Power per unit area of 4W/m2 can be achieved– Algae at sea – how to supply CO2 (growth rate drops

100 fold without CO2 enrichment)– Algae can be used for Hydrogen production – circa

4.4W/m2

Overall Benefits and Impacts of Bioenergy

• Benefits– Carbon Neutral– Can be harvested in liquid

form for transport applications

– Large choice of biomass materials that can be matched with local climate

– Sustainable energy source (replenishment from sum)

– Security of supply– Waste materials can be

used– Byproduct of some

processes is fertilizer

• Impacts– Low energy per unit area– Social impact of replacing

good food producing land with energy crops

– Only viable as a component of an overall energy delivery system

– There are some emissions: reductions in CO2 emissions relative to fossil fuels but particulates and NOx remain a problem.